Deciding whether to kill one person to save five is a true brain teaser. A study in the March 26 Journal of Neuroscience describes the neural tug-of-war that results in a moral decision.

Cognitive neuroscientists Amitai Shenhav of Princeton University and Joshua Greene of Harvard University asked 35 people to weigh in on 48 wrenching scenarios while undergoing functional MRI brain scans. The researchers used scenarios akin to the famous trolley choice: The hypothetical dilemm...